I think someone else said what it actually is in another comment. It's functionally identical 90℅ of the time for me anyway,and I use CLI and vim on it.

They're both UNIX-like, i.e. they both implement the POSIX specification and are therefore in many ways compatible.

But yeah, modern macOS is more directly derived from the original UNIX operating system.
Linux was instead implemented from scratch to be compatible with UNIX.